Problem: $ B = \left[\begin{array}{rrr}2 & 0 & 1 \\ 2 & 1 & 0 \\ 1 & 0 & 1\end{array}\right]$ What is $ B^{-1}$ ?
Answer: $ = \left[\begin{array}{rrr}1 & 0 & -1 \\ -2 & 1 & 2 \\ -1 & 0 & 2\end{array}\right]$